Digital control of low-power switching converters allows for numerous benefits including the ability to use digital design tools, flexibility in transferring to different implementation technology, and low sensitivity on external influences.
However, in battery-powered handheld devices, such as cell phones, digital still cameras (DSC), and personal data assistants (PDAs), analog controlled dc-dc switching converters are predominantly used. Among the main obstacles for successful digital implementation are lower switching frequency, compared to analog solutions, and the absence of low-power digital architectures that can support pulse-frequency modulation (PFM). The PFM results in significant efficiency improvement when the output load of a switch-mode power supply (SMPS) is light. In the handheld devices, to extend the battery life, the PFM is usually used when the supplied devices perform simple processing tasks or operate in stand-by mode.
In recent publications several low-power solutions that either support only digital pulse-width modulation control or combine a digital-pulse width modulator (DPWM) and an analog PFM have been presented.
The problem with analog implementation is that the analog controllers require relatively long design process and need to be almost completely redesigned each time implementation technology changes. As such, they are not suitable for monolithic integration with fast changing digital hardware, on which the majority of portable devices are based. Moreover, in the latest CMOS processes not all functional blocks of analog controllers can be implemented due to very limited supply voltages.
On the other hand, some DPFM are not suitable for low power switching converters either. To create a pulse-frequency modulated (PFM) signal, often characterized with short on and long off time intervals, some realizations use a high frequency clock and employs power-inefficient counters which make them unsuitable for low-power applications, where the overall efficiency of the SMPS is significantly affected by the power consumption of the controller.